1. Field of the Invention
This invention relates generally to the field of leak detection, wherein a leaking fluid causes an impedance change in a sensing cable and the impedance change is detected as an indication of the leak.
2. Related Art
A variety of chemicals (e.g., oils, crude oil, refined petroleum products, gasoline, kerosene, organic solvents, and the like) are stored in underground tanks and transported through buried pipelines. Leakage from these tanks and pipelines can contaminate ground water and cause extensive environmental damage. Further, leaks are difficult to detect and often are not detected until extensive environmental damage has already occurred.
One method of detection has been to run a cable adjacent to the underground pipeline or tank. The cable includes a pair of conductors (e.g., coaxial or twin lead) with a permeable insulation disposed therebetween. If a leak occurs, the chemical will permeate the insulation and will cause a change in its dielectric properties such that the electrical characteristics between the conductors is changed. This will result in an impedance change that can be sensed at a remote end of the cable.
A common means for performing such sensing is time domain refelectometry (TDR) wherein an electrical pulse is propagated down the cable and any reflections are received and observed. An additional means which may be used is capacitance measuring.
Capacitance measuring has been used to locate faults in electrical communication cables. If the measured capacitance of a cable is compared with a known distributed capacitance per unit length for that cable, the length of the cable or the location of an open fault (i.e., a break) may be determined. In a similar manner, capacitance measuring could be used to sense a change in the capacitance of a cable due to permeation of a liquid chemical.
A known means for measuring capacitance is to charge a cable with a constant current to a predetermined voltage and to measure the charge time. The charge time can then be related to the value of the unknown capacitance. U.S. Pat. No. 3,452,272 to Collins et al., U.S. Pat. No. 3,761,805 to Dornberger, and U.S. Pat. No. 4,103,225 to Stephens provide examples of constant current capacitance measuring systems.
While these systems may be relatively simple, they tend to yield poor accuracy (i.e., the systems suffer from measurement error) and/or require frequent calibration. A substantial amount of error is caused by variations in the constant charging current, which is caused by variations in the power supply voltage and variations in circuit component values.